When verifying the proper operation of circuitry components, test equipment is often used to generate test signals that are provided to the components being tested. In order to ensure accurate verification, the characteristics of these test signals have to be controlled. Examples of these signal characteristics are signal type (e.g., DC, AC, sine wave, square wave, etc.), signal frequency, frequency sweep rate, signal amplitude, and amplitude sweep rate.
Variable attenuators are often used to control signal amplitude. Examples of these variable attenuators include voltage-variable analog attenuators, PIN-diode attenuators, and digital attenuators (e.g., switched-bit digital attenuators).
Voltage-variable analog attenuators typically include one or more field effect transistors that are controlled (via their gate voltage) to act like variable resistors in a series or a shunt configuration. PIN-diode attenuators typically forward bias a PIN diode so that they function as current-controlled resistors.
Voltage-variable attenuators and PIN-diode attenuators may be controlled digitally with the combination of a DAC (i.e., digital-to-analog converter) and an operational amplifier. Unfortunately, even with digital control, voltage-variable and PIN-diode attenuators provide non-linear performance, require time and cost to calibrate, and increase the parts count per attenuator unit.
While digital attenuators allow for direct digital control, these attenuators are typically too coarse, in that the highest-resolution achievable (i.e., the smallest amount of attenuation selectable) is between 0.25 and 0.50 decibels.